The present disclosure generally relates to power factor correction.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Power factor is a ratio of real power flowing to a load to apparent power. Power factor may be described as a number between 0 and 1 or expressed as a percentage. It is desirable to have the power factor be closer to 1 or 100%.
Two factors may affect the power factor. A displacement factor is when a current waveform is not in-phase with a voltage waveform. A distortion factor is when the current waveform is not sinusoidal; that is, distortion may be present in the current waveform. Power factor correction may be used to correct these two factors.
FIG. 1 shows two waveforms illustrating effects of the displacement and distortion factors. A voltage waveform 102 and two current waveforms 104a and 104b are shown. The two current waveforms 104a and 104b illustrate the displacement factor and the distortion factor separately.
Current waveform 104a shows the displacement factor. A phase difference 1 exists between voltage waveform 102 and current waveform 104a. Current waveform 104a is thus delayed with respect to voltage waveform 102.
Current waveform 104b shows the distortion factor. Current waveform 104b is in phase with voltage waveform 102; however, current waveform 104b is distorted. For example, total harmonic distortion (THD) is present.
A combination of the displacement factor and distortion factor causes the power factor to be lower. Power factor correction is used to shape the current waveform to make it sinusoidal and in-phase with voltage waveform 102, which raises the power factor.
During power factor correction, it may be desirable to limit the maximum current in an input circuit. If the current is not limited, a system may be damaged. For a given system, input power is given by the root mean square (rms) of the input voltage Vin rms multiplied by the root mean square of the input current Iin rms. The range of the input voltage Vin rms is typically 85V-277V. The maximum current occurs at the minimum input voltage Vin rms of the range for constant input power. FIG. 2 shows a graph 200 of input power vs. input voltage Vin rms for constant input current. As shown, as the input voltage Vin rms increases, the input power increases. At a point 202, the input voltage Vin rms is at its lowest and the current is expected to be at its highest. The maximum current limit is thus set at a percentage above the current that occurs at point 202 because this is expected to be the maximum current over the range of voltages for the input voltage Vin rms. The maximum current limit is constant and does not change. Because the limit is constant, this may lead to high input power over the voltage range when the current limit is reached at every switching cycle of a switched mode power supply.